The present invention relates to a process for generating a mixture of phosphine and diluent gas or gases, wherein a hydrolysable metal phosphide selected from the group consisting of magnesium phosphide, aluminium phosphide and calcium phosphide is contacted with liquid water in a generating space, whereby the metal phosphide is hydrolysed to release phosphine which is withdrawn from the generating space and, where applicable, diluted from the time of its generation to its reaching its locality of use with a diluent gas to a composition which is non-ignitable under the conditions of use. The invention also provides a novel generator suitable for carrying out the process.
Phosphine gas is a highly toxic and flammable gas used in large quantities in pest control, and in particular for the fumigation of agricultural bulk commodities, such as grain and grain products. Phosphine gas generation is also subject to some peculiarities giving rise to special problems which do not apply to the generation by hydrolysis of other gases, e.g. the well-known generation of acetylene gas by hydrolysis of calcium carbide as disclosed e.g. in British patent specifications 472 970 (Haworth), 776,070 (Union Carbide) and 291,997 (Haworth).
In the case of phosphine gas generation there has always been the problem that prior art hydrolysable technical grade metal phosphides contained impurities which on hydrolysis liberated autoignitable phosphine homologues, phosphine derivatives, organophosphines, diphosphines or polyphosphines. This circumstance has created a strong prejudice in the art against what the present application proposes in what follows.
Traditionally compositions containing hydrolysable metal phosphides, in particular aluminium, magnesium and calcium phosphides have been used for this purpose, applied either in sachets or other dispenser devices or as moulded bodies (pellets or tablets).
In either case, the traditional compositions have always been compounded with various additives to a) reduce the reactivity of the metal phosphide when exposed to water in vapour or liquid form and b) to depress their tendency to autoignite. (Rauscher et al U.S. Pat. No. 3,132,067, Friemel et al U.S. Pat. No. 3,372,088, Friemel et al U.S. Pat. Nos. 4,421,742 and 4,725,418, Kapp U.S. Pat. No. 4,347,241). In spite of these expedients, these prior art products remained dangerous substances, involving fire and explosion hazards which had never been fully overcome if the products are handled inexpertly and stringent safety precautions are neglected. The degree of safety also depends on the experience of the manufacturer and quality control. The traditional manner of using these products in bulk commodity fumigation is to introduce the compositions into the storage means (e.g. silos, shipholds) as such. In the case of pellets or tablets, these are usually introduced into the bulk commodity itself. This practice is nowadays criticised because of the resultant contamination of the bulk commodities with the residues of the decomposed tablets or pellets.
If prior art compositions are apportioned in sachets, bag-blankets, bag-chains or similar dispensers, the purpose is to divide the composition into small individual portions in order to reduce the hazards of large local accumulations of gas and heat build up and at the same time prevent direct contact of the compositions with the commodities. These devices must, after completion of the fumigation, be retrieved from the storage or like facility where the fumigation has taken place. This is often difficult and cumbersome. The spent devices must then be disposed of, a matter which nowadays may cause problems.
All these and other prior art fumigation means and their traditional methods of application suffer from the drawback that once the devices have been introduced into the silo or other storage space and once the fumigation has commenced, there is usually very little that can be done to influence or even monitor the further progress of the fumigation. In particular, if the composition should accidentally be deposited in a wet spot inside a grain store, this will neither be noticed in time, nor can the resultant dangerous situation be corrected. A fumigation of this type once commenced, can normally neither be stopped nor (usually) be decelerated or accelerated.
To overcome these shortcomings to some extent new processes have been developed wherein tablets and pellets or the aforesaid sachets, bag-blankets, bag chains or similar dispensers are distributed e.g. on the surface of the bulk commodity and to then apply recirculation of the gas content of the silo, storage space or shiphold; see U.S. Pat. Nos. 4,200,657 (Cook), 4,651,463 and 4,756,117 (Friemel) and 4,853,241 and 4,729,298 (Dornemann).
In those cases contamination, if any, is more localised and the spent dispensers are more readily retrieved, although these are still inaccessible whilst the process is in progress. The aforesaid climatic and humidity limitations still usually apply. The time taken for achieving a scheduled concentration of phosphine throughout the storage space still depends on the rate at which the metal phosphide composition is hydrolysed under prevailing circumstances. If the applied circulation is too slow or ceases, e.g. due to a power failure, undesirable concentrations of phosphine may accumulate.
It has been recognised that it would be highly advantageous if it were possible to transfer the generation of phosphine gas to a locality outside the fumigation space whereafter the gas could then be fed into the commodity or storage facility in a controlled manner. However, because of the conceived and real risks inherent in phosphine gas and phosphine-releasing compositions, very little real progress has been made in this regard.
Thus the use of bottled PH.sub.3, produced by one or other undisclosed industrial process, has been proposed in U.S. Pat. No. 4,889,708. Again, in order to prevent autoignition once the gas is released into air and the mixture of air and gas is used as a fumigant, it was considered necessary to bottle the PH.sub.3 highly diluted with an inert carrier gas such as CO.sub.2 or N.sub.2. According to U.S. Pat. No. 4,889,708, the PH.sub.3 concentration in the bottled gas is to be 1.8 to 3% by weight. The storage and transport of this highly diluted phosphine gas involves considerable logistics problems, besides being very expensive.
It also involves the grave risk that in the event of an accident on site, in transport or in storage or in the event of leaking bottles, e.g. due to defective or not properly closed valves, a gas cloud, albeit not readily flammable, is formed which is highly toxic and which, because it is heavier than air, can accumulate in low-lying areas or in cellars or the like.
U.S. Pat. No. 5,098,664 discloses a recent attempt to overcome the prejudice existing in the art against the generation of phosphine gas in an external generator apparatus, wherein relatively large concentrated batches of metal phosphide are hydrolysed by the passage therethrough of controlled amounts of water vapour dispersed in humid air, the air serving as a carrier gas. This proposal still suffers from certain potential shortcomings. That disclosure teaches interrupting the hydrolysis in the event of operational failures by displacing the humid air in the generator space by an inert fluid, (liquid or gas). The recirculation type of process has similarly been improved in accordance with European patent application 9 114 856.8 (Degesch GmbH; published after the priority date of the present application) in that the hydrolysis of the solid metal phosphide compositions takes place outside the space containing the commodities to be fumigated in a hydrolysis chamber through which the circulatory gas flow is passed. Again, in the event of problems necessitating the interruption of gas generation, inert gas is admitted into the hydrolysis chamber to displace the humid air. In both the aforesaid cases there can be a considerable delay before humidity which has already partly reacted with the metal phosphide is fully consumed so that no further generation of phosphine takes place. This prolonged delayed release of phosphine can be explained by the following reactions. Normally the following reaction predominates when magnesium phosphide is exposed to humidity: EQU Mg.sub.3 P.sub.2 +6H.sub.2 O.fwdarw.3 Mg(OH).sub.2 +2PH.sub.3
However, if the admission of humidity is interrupted, the already formed magnesium hydroxide continues to react with not yet hydrolysed magnesium phosphide as follows: EQU 3 Mg(OH).sub.2 +Mg.sub.3 P.sub.2 .fwdarw.2PH.sub.3 +6 MgO
This latter reaction, because of the solid nature of the reactants, is slow and continues over a prolonged period. The above phenomenon also applies to other metal phosphides, e.g. aluminium phosphide.
Complete control of all aspects of the aforesaid generator and process is nevertheless feasible but is complex and expensive.
A number of more recent similar proposals are disclosed in PCT application WO 91/19671. Some embodiments again involve reaction of metal phosphide compositions with water vapour, and these embodiments are subject to the abovementioned problems. In most embodiments the phosphine is released into air, and the risk of ignitable mixtures of phosphine and air being formed cannot be excluded.
In some other embodiments prior art tablets (as described further above) are dropped periodically one by one at a controlled rate into a water bath inside a generating space. The compositions, being in the form of compressed bodies, namely tablets, are specially compounded to reduce their reactivity. These tablets take a relatively long time to decompose when dropped into water, even if the water is heated, as proposed in this prior art. Accordingly this prior art process and apparatus suffers from the drawback that the gas generation is relatively slow and can only be accelerated by increasing the rate of feeding tablets into the water. This in turn means that the amount of metal phosphide submerged in the water bath at any one time is relatively large, and accordingly, if it becomes necessary to stop the gas generation because of some operational failure, it will take a long time before the gas generation stops, and large volumes of phosphine gas are generated during that period which have to be disposed of in some way or another. In most embodiments the phosphine is released into air and the risk of ignitable mixtures of phosphine and air being formed cannot be excluded. In addition, the gases released by such prior art composition, when dropped into water have a greater or lesser tendency to autoignite.
Moreover the prior art compositions used in that process release paraffin wax or other hydrophobic and other additives into the water bath. The hydrophobic contaminants in particular float on the water surface and interfere with the smooth progress of the process by forming emulsions and entrapping metal phosphide particles and generally contaminate the water in the apparatus and the apparatus itself, causing a disposal and cleaning problem. These problems also arise from the proposals in PCT application WO 93 25075 wherein an extrudable paste of the metal phosphide and a water-immiscible, grease-like substance is squeezed into water in a generator space.
Accordingly there exists a need for a process and apparatus of the type set out in the aforegoing which does not suffer from the aforesaid disadvantages or wherein these disadvantages are substantially mitigated. In particular there exists a need for a process and apparatus permitting the safe production of phosphine-containing gases in an environmentally friendly manner, with a minimum of disposal problems of potentially harmful metal phosphide residues and/or oily or greasy contaminants. Such process and apparatus should also be easily controllable in the case of operational failures, e.g. electrical power failures, and may indeed in certain embodiments be operable independently or substantially independently of any external electrical power supply.
There also exists a need for a metal phosphide composition suitable for carrying out the process and which will deliver a phosphine gas having no or no appreciable tendency to autoignite.
The aforesaid prior art compositions suffered from the disadvantage that the phosphine gas released therefrom on hydrolysis has a greater or lesser tendency to autoignite. This problem has been linked to the hitherto unavoidable presence in the metal phosphide of contaminants which on hydrolysis liberate autoignitable phosphine homologues, phosphine derivatives, organophosphines, diphosphine or polyphosphines. Because the presence of these contaminants was considered unavoidable, the incorporation of the additives in accordance with the above cited prior art was considered unavoidable. In addition, the aforesaid forms of presentation as pressed bodies or in dispensers such as sachets were intended to slow down greatly the hydrolysis reaction, to avoid heat build-up and build-up of ignitable or explosive gas accumulations and concentrations.
U.S. Pat. Nos. 4,331,642 and 4,412,979 to Horn et al and UK patent application 2097775 by Degesch GmbH disclose a process purported to result in the formation of magnesium phosphide free of such contaminants by the reaction of magnesium and yellow phosphorus at a temperature between 300.degree. and 600.degree. C. In spite of these claims, it was considered necessary to compound this magnesium phosphide with large amounts of additives and resinous binder in the form of so-called "plates" as described in German patent 2002655.
This material has, in the past, invariably been phlegmatised immediately after its formation by impregnation and coating with a hydrophobic substance, preferably hard paraffin in amounts of about 1 to 4%, preferably 2 to 3.5%, before any further handling or before storage prior to use in the manufacture of compositions for pest control purposes, such as the aforesaid "plates". For the aforesaid reasons the pure metal phosphide, such as the highly reactive magnesium phosphide, in its unphlegmatised form was never as such in the past made available to the public.
Although these plates have been very successful commercially and play an important role in the art, they have to be handled with the same great care as other conventional metal phosphide preparations, inter alia because of the risk of autoignition on contact with liquid water, for hitherto unknown reasons.